In electrophotographic photoreceptors, inorganic photoconductive materials, such as selenium, cadmium sulfide , and zinc oxide, have hitherto been employed. As compared with the inorganic photoconductive materials, organic photoconductive materials, such as polyvinylcarbazole, oxadiazole and phthalocyanine, have met difficulty in putting to practical use due to their low sensitivity, though advantageous from the viewpoint of environmental conservation and high productivity. Several approaches for increasing sensitivity of organic photoconductive materials have been proposed to date. Among the proposals, a so-called separate function type photoreceptor having a laminate structure composed of a charge generating layer and a charge transporting layer is an effective one and has already been put to practical use.
An electrophotographic photoreceptor, as a matter of course, is required to have prescribed sensitivity, electrophotographic characteristics, and optical characteristics in conformity with an electrophotographic process adopted. In particular, an uppermost layer (i.e., the layer farthest from a conductive substrate) of a photosensitive layer in a repeatedly usable photoreceptor is demanded to have durability against electrical or mechanical outer force directly imposed thereto through steps of corona discharge, toner development, transfer to paper, and cleaning. More specifically, the uppermost layer (hereafter, sometimes referred to as surface layer) should be resistant against reduction in sensitivity or initial potential or increase in residual potential due to deterioration by ozone generated during corona charging and against wear or scratching due to surface sliding.
Further, adhesion of paper dust to a photoreceptor due to contact with paper is one of causes of image diffusion in a high temperature, and remaining of a toner on a photoreceptor due to toner filming or insufficient cleaning seriously impairs image quality. Therefore, it has been demanded to form a surface layer which is hardly contaminated with paper dust or toner particles and also which is easily cleared of such contaminants.
In order to meet the above demand, various improvements have been added. For example, it is known that polycarbonate resins of bisphenol A type, polystyrene resins, polyacrylic resins, and the like are known to serve as a binder resin effective to improve charging characteristics, photosensitivity, residual potential, and durability against repeated use.
However, when used in an electrophotographic copying machine, the electrophotographic photoreceptors using these resins as a binder tend to suffer from scratches or wear through sliding with a magnetic brush or a cleaning blade.
On the other hand, workability in copying operation and quality of the resulting image are also influenced by uniformity of the coating layer(s) inclusive of the surface layer of a photoreceptor. Accordingly, the coating layer of a photoreceptor is required to have a uniform thickness and a smooth and homogeneous surface for assuring high image quality and satisfactory workability. In other words, surface defects, such as orange peel, pin holes, coating streaks, solvent cracks, etc., attributed to the composition of a coating composition or coating or drying conditions give rise to serious problems for copying characteristics and the production technique.
Use of a surface active agent is known effective to improve surface properties or slipping properties. In a suspension coating system, it is also effective to improve dispersibility and dispersion stability of the suspensoid. In a solution coating system, it is effective to improve solubility of the solute and coating properties of the solution. However, unless properly selected, a surface active agent often causes troubles, such as poor adhesion between layers, disorders due to denaturing, or poor moisture resistance.
In order to overcome the troubles associated with the use of a surface active agent, improvement of a polycarbonate resin of bisphenol A type (see JP-A-60-172045, the term "JP-A" as used herein means "an unexamined published Japanese patent application"), use of a siloxane-blocked copolymer of a polycarbonate resin (see U.S. Pat. No. 5,080,987) and a combined use of a polycarbonate resin with a comb type silicone-grafted polymer (see JP-A-62-208053) have been suggested. However, any of these binder resins still fails to provide sufficient surface strength and smoothness, and the resulting photoreceptor has poor resistance to wear and scratching and undergoes reductions in image quality on repeated use and in sensitivity due to wear of the surface.